Interferometers based on laser beams are used to make highly accurate displacement measurements, such as required in the control of wafer steppers used in integrated circuit (IC) manufacturing. In a distance-measuring laser interferometer, light from a laser source is split into two beams. The reference beam is reflected from a stationary reference mirror, while the measurement beam is reflected from a moving measurement mirror. The beams are recombined at a detector. The optical intensity of the combined beams depends on the difference in optical length between the reference and measurement paths. Measurements of the optical path to an accuracy of a fraction of the wavelength of the laser are routinely obtained.
Distance-measuring interferometers are typically divided into DC and AC interferometers. In a DC interferometer, the laser emits a single frequency. Only when the measurement mirror is moving is the interference signal time-varying. When the measurement mirror is stationary, the interference signal is a constant. Unfortunately, disturbances such as laser power drift and electronic noise can be easily misinterpreted as a motion signal, especially when the measurement mirror is stationary.
In an AC interferometer, the laser emits two optical frequencies with orthogonal polarizations. The two optical frequencies differ by a small amount. One of the beams is directed along the reference path while the other is directed along the path to be measured. The frequencies are separated with a polarization-dependent beam splitter, with one frequency going to the reference mirror and the other going to the measurement mirror. When he beams are recombined, a beat frequency at the difference in optical frequencies is created. When the measurement mirror moves, the beat frequency shifts because of the Doppler shift induced by the motion. In this arrangement, the distance measurement is obtained by taking the difference of the frequency observed when the measurement mirror is moving and the frequency when both mirrors are stationary. This later frequency is obtained by directing a portion of the laser's output at an appropriate detector to generate the beat frequency. Since only the component of the noise within the frequency band between the reference beat frequency and the beat frequency observed when the mirror is moving can interfere with the signal, the effects of noise are substantially reduced in AC interferometers.
Thus, the detector generates an AC signal when the measurement mirror is stationary as well as when it is moving. It is easier to reject noise with a time-varying signal than with a constant one. Therefore, AC interferometry is more accurate than DC due to its superior ability to reject noise
The distance measured by observing the above-described difference in frequencies, or by counting fringes in the case of a DC interferometer, is the difference in the optical path between the reference arm of the interferometer and the arm containing the moving mirror. In most cases, the parameter of interest is the difference in physical distance. The physical path length is the optical path length divided by the average index of refraction of the air on the path traversed by the light beams. Hence, the interferometric measurement must be corrected for the index of refraction of the air along the path. In practice, the air along the measurement path may be turbulent, particularly in the region surrounding the wafer stage of a stepper. The index of refraction depends on the local air density along the path. Hence, unless the index of refraction is known on the actual path at the time the measurement is being made, errors will be made in the conversion from optical path length to physical distance. As the feature sizes in circuits shrink, the errors resulting from air turbulence can lead to serious position-measurement errors. Hence, methods for measuring the index of refraction simultaneously with the optical path length have been proposed.
One method for simultaneously determining the density of air and the physical path length is to use the measured relationships between the index of refraction of air, the density of air, and the optical path length. Since the index of refraction changes with wavelength, the average density, and hence, index of refraction can be deduced by measuring the optical path length at two or more wavelengths.
Measurement systems based on measuring the optical path length at two widely separated frequencies are known to the art. For example, Lis (U.S. Pat. No. 5,404,222) describes a system in which two lasers are utilized to measure the optical path length at different frequencies. The system taught by Lis requires a much more complex optical system than that utilized in a conventional AC interferometer. This system requires 3 wavelengths, and multiple distance measurements to correct for air turbulence. In addition, the system has a poor signal-to-noise ratio because it relies on non-resonant second harmonic generation to provide the multiple wavelengths. The system also relies on expensive optical techniques to generate a correction signal.
Broadly, it is the object of the present invention to provide an improved AC interferometer.
It is a further object of the present invention to provide an interferometer that automatically compensates for turbulence along the measured optical path.
It is a still further object of the present invention to provide an interferometer that is less complex than prior art interferometers that compensate for turbulence.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.